With the recent progress in telecommunication technologies utilizing computers and the like, there is an ever-increasing need for large-volume information transmission. One of the most important means for transmitting such large-volume information are optical fibers. In optical fiber based transmission, optical signals are transmitted through an optical fiber. The optical fiber is formed of a glass material because of its advantages such as transmitting a wide range of wavelengths, having a high transparency, suited for manufacturing a long fiber, and being cost-effective.
Construction of an optical fiber transmission system requires elements such as a light source, an optical receiver, an optical signal generator, an optical switch/coupler, and an optical connector for connection with a transmission optical fiber. The electro-optic effect (optical non-linearity) is utilized especially for optical switching elements such as the optical signal generator and the optical switch. The optical non-linearity is generated by non-linear polarization induced in material by light (electromagnetic waves), and intensity and direction of light transmitted through the optically non-linear material is changed by controlling factors such as electric field strength applied to the material. By use of such optical non-linearity, various optical functional elements such as optical switching elements are formed.
Further, the light introduced to the optical fiber is switched on/off by such optical functional elements based on the information to be transmitted, to thereby achieve light modulation. The optical signals are then demodulated at the receiving side, completing transmission of information through the optical fiber.
A plurality of information items can simultaneously be transmitted if light beams of a plurality of wavelengths are guided through the optical fiber. If information is transmitted at different points in time, distinction between the different information items can surely be made by utilizing different wavelengths. Therefore, a need exists for guiding light beams of a plurality of wavelengths through a single optical fiber.
In conventional optical transmission systems, a wavelength of light is basically selected by an optical filter, and isolation of a prescribed wavelength requires a separate filter. A conventional optical switch is designed to switch on or off a light beam of a single wavelength, but not to switch light beams of a plurality of wavelengths. Therefore, if light of a plurality of wavelengths is introduced, wavelength switches are provided for the respective wavelengths.
However, for transmitting light of a plurality of wavelengths through a single optical fiber, or for the means for collectively handling and switching light beams of a plurality of wavelengths, selecting and handling light beams of a plurality of wavelengths would be extremely advantageous and contribute to realizing a simplified system. Therefore, an optical functional element which allows selecting and handling light of a plurality of wavelengths is desired.
Crystalline material such as LiNbO.sub.3 and BaTiO.sub.3 are used as the optically non-linear material for implementing such optical functional element because currently the crystalline material is the only material that presents sufficient optical non-linearity.
Meanwhile, it is desirable to use glass material to form optical functional elements such as optical switches in terms of a stable connection with a glass optical fiber, low loss of the transmitted light, low cost, and a wide range of transmitting wavelengths. However, glass material basically does not have optical non-linearity and, therefore, cannot be used for this purpose.
Attempts to introduce optical non-linearity to a glass material have been made. For example, UV-excited poling carried out by irradiating a glass material with ultra-violet radiation while applying a high electric field of approximately 10.sup.6 V/cm is disclosed in "ELECTRONICS LETTERS 30.sup.th," March 1995, Vol. 31, No. 7, pp. 573-574.
UV-excited poling is believed to provide glass material with similar optical non-linearity to crystalline material, allowing the glass material to be suitably employed in optical functional elements.
A grating element is a known element utilizing an optical fiber having a core where portions with different refractive indices are formed. In a grating element, wavelengths of transmitted light and reflected light are varied with intervals in the grating. The grating element is used for devices such as temperature sensors.
Proposals have been made to form grating elements by irradiating a glass optical fiber having a core doped with germanium (Ge) with ultra-violet rays in a prescribed intensity pattern. According to this method, prescribed ultra-violet radiation is irradiated to change the refractive index at the irradiated portion, and thereby form a refractive index grating in the optical fiber. Fabrication of such grating element is proposed in, for example, JP-T2-62-500052.
As described above, the UV-excited poling method conventionally proposed can impart non-linearity to the glass material. However, this method imparts optical non-linearity only to a certain range of the optical fiber core, and therefore it only suggests its possibility of its application as an optical functional element.
In addition, the above conventional grating element has a grating which exhibits a change in refractive index. As a result, such a grating element does not have optical non-linearity and cannot form a variety of elements that utilize electro-optical effects.
Fabrication of grating elements by utilizing UV-excited poling is proposed in "Optical Fiber Communication Conference; OFC '95 Postdeadline Paper PD6," published in 1995. This document, however, only makes a general proposal on forming a grating element by utilizing UV-excited poling, and does not specifically disclose the formation.